hayden_craig95•7d agoMost Upvoted

Piston ring end gap specs from the manual vs. what old heads swear by

I spent years setting rings exactly to factory spec because I figured the engineers knew best, but a veteran engine builder in Phoenix showed me a 0.003" tighter gap on a 350 Chevy that ran cleaner and never smoked. Anyone else find that the book numbers get you by but the real tricks come from seat time?

3 comments

3 Comments

karenb977d ago

A buddy of mine built a 351 Windsor for his '69 Mustang and set the gaps dead nuts on from the Ford manual. Thing ran okay but always had a tiny puff of blue smoke on startup, drove him crazy. He took it to an old guy named Sal who builds marine engines down in Florida, guy just laughed and said "your rings are too loose, kid." Sal had him pull the pistons and set the top ring gap 0.004" tighter on each one, regapped the second ring a little looser too. Motor fires up instantly now, no smoke at all, and pulls harder on the top end than it ever did before. My buddy still swears by that guy's method, says the manual numbers are just a starting point for safe mass production.

jakeb814d ago

Did Sal say why tighter on top and looser second ring makes the difference?

sanchez.mary7d ago

The part about "your rings are too loose, kid" really hit home with me. I had a similar experience with an old Ford 302 I was rebuilding, where the book said .018 for the top ring and I set it exactly there. It ran fine but had that same annoying blue puff on cold mornings, just like your buddy's Windsor. A local machinist named Pete told me to try .015 on the top and .020 on the second, and it was like a different engine altogether-no smoke, better throttle response, and it actually used less oil between changes. I've never gone back to straight factory numbers since then, especially on older blocks where the cylinders might have a little wear or different metallurgy than what the engineers assumed.